Carbonyl compounds refer to compounds having >C═O function group, such as aldehydes or ketones. The reaction of carbonyl compound with hydroxylamine is a main process for the syntheses of the corresponding oxime compound.
For example, in the case of cyclohexanone oxime, cyclohexanone oxime is a key intermediate for producing ε-caprolactam which is an important raw material in organic chemical industry mainly used as a monomer for synthetic fibers and-engineering plastics (e.g., nylon-6). About 91% of caprolactam are produced industrially via a technique route with cyclohexanone oxime as intermediate product, in which cyclohexanone oxime is produced by the reaction of cyclohexanone with hydroxylamine (used in its sulfate or phosphate form). This process for the production of cyclohexanone oxime has a complex technology with multiple process steps and high investment in equipment, and it also has a problem in corrosion and pollution due to use or production of NOx, SOx and the like.
In the early 1980's, in U.S. Pat. No. 4,410,501, Taramasso (Italy) disclosed a novel type of catalyst material—titanium silicalite having an excellent function for selective oxidation of hydrocarbons, alcohols, phenols and the like (EP 0230949, U.S. Pat. No. 4,480,135, U.S. Pat. No. 4,396,783). It has been commercialized to use it for the preparation of catechol and hydroquinone by the selective oxidation of phenol with hydrogen peroxide.
EP 0208311, EP 0267362, EP 0496385, EP 0564040, etc. sequentially disclose a novel process for preparing cyclohexanone oxime in one step by ammoximation of cyclohexanone with ammonia and hydrogen peroxide catalyzed by titanium silicalite. This novel process features in mild reaction conditions, high yields of desired products, more efficient process, lower investment in equipment, reduced amount of wastes and environmental friend.
Furthermore, EP0347926 discloses that the ammoximation of cyclohexanone is carried out by using a catalyst in which titanium dioxide is dispersed on silica, also exhibiting a relatively good catalytic performance; both J. Le. Bars et al., Appl. Catal. A 136(1996) p. 69 and P. Wu et al., J. Catal. 168 (1997) p. 400 report that other types of Ti-containing crystalline silicate, such as Ti-ZSM-48, Ti-β, Ti-MOR and the like, which all exhibit relatively good catalytic performance for the ammoximation of a variety of aldehyde or ketone compounds.
As the ammoximation of cyclohexanone is investigated intensively in preparing cyclohaxnone oxime, deactivation of various titanium-containing catalysts, mainly titanium silicalites, in this reaction has been increasingly focused.
EP 0496385 discloses that it is necessary to remove off the deactivated catalyst periodically, which is to be replaced by a fresh catalyst make-up in order to maintain the desired catalytic activity during the reaction.
U.S. Pat. No. 5,498,793 discloses a process for the production of oximes which comprises ammoximation of a carbonylic compound selected from acetophenone and cyclododecanone with hydrogen peroxide and ammonia in the presence of a catalyst based on silicon, titanium and oxygen and a cocatalyst consisting of amorphous silica. The cocatalyst added in the said process can increase the yields and conversions in the ammoximation of acetophenone and cyclododecanone, but does not solve the deactivation of the main catalyst.
CN1345718A discloses a process for preparing oxime from carboxyl compound, hydrogen peroxide and ammonia, involving the addition of cocatalyst containing acidic solid into the reaction system so as to increase the conversion rate of the ammoximation of the carbonyl compound. However, the acidic solid cocatalyst added in the said process does not solve the deactivation of the main catalyst.
G. Petrini et al., Stud. Surf. Sci. Catal. 68(1991) p. 761 identifies the three main deactivation processes of titanium silicalites in the ammoximation of cyclohexanone: (1) slow dissolution of the framework (silicon) with accumulation of Ti on the external surface of the remaining solid, (2) direct removal of Ti from the framework and (3) pore filling by by-products. The first two are due to the basic reaction medium in the presence of ammonia, which results in silicon dissolution away from the titanium silicalite framework. Since only Si is removed from and Ti remains in the catalyst, Ti content of the catalyst relatively increases and the degree of crystallinity of the catalyst tends to decrease. The literature further shows that, although there is only a small amount of silicon (ppm) dissolved in the reaction stream, the silicon dissolution during the long term running will result in a continuous decrease in the amount of the titanium silicalites in the reaction system. The weight of catalyst recovered will be lower than that of the starting one. Under extreme conditions, the recovery of catalyst is 35% only.
In the book Selective Oxidation by Heterogenous Catalysis (2001, p. 112), it is shown that dissolution of silicon in Ti-containing crystalline silicas caused by ammonia is the primary factor leading to deactivation of the catalyst during ammoximation of cyclohexanone. Since ammonia is an indispensable raw material in the ammoximation of cyclohexanone, the problem caused thereby is inevitable. Although this problem has been confirmed, no relevant technical solutions solving the problem have been reported yet. Similar problem is also present in reaction systems of ammoximation of other aldehydes and ketones.
In the ammoximation of carbonyl compounds mentioned above, the dissolution loss of silicon from the catalysts will result in disadvantageous effects, such as reducing the stable operation time of catalyst and decreasing the recovery of catalyst. Nevertheless, although the silicon dissolution in the Ti-containing crystalline silicas caused by ammonia is the primary factor for the deactivation of catalyst, adding into the system the acidic solids, such as solid silica gel, as stated in CN1345718A cannot solve the problem as to the deactivation of catalyst.